Low Back Moments Research Articles

The effects of ergonomic interventions on low back moments are attenuated by changes in lifting behaviour

This study investigated the effects of ergonomic interventions involving a reduction of the mass (from 16 to 11 and 6 kg) and an increase in the initial lifting height (from pallet height to 90 cm above the ground) of building blocks in a mock-up of an industrial depalletizing task, investigating lifting behaviour as well as low back moments (calculated using a 3-D linked segment model). Nine experienced construction workers participated in the experiment, in which they removed building blocks from a pallet in the way they normally did during their work. Most of the changes in lifting behaviour that were found would attenuate the effect of the investigated interventions on low back moments. When block mass was reduced from 16 to 6 kg, subjects chose to lift the building block from a 10 (SD 10) cm greater distance from the front edge of the pallet and with a 100 (SD 66) degrees/s2 higher trunk angular acceleration. When initial lifting height was increased, subjects chose to shift the building blocks less before actually lifting them, resulting in a 10.7 (SD 10) cm increase in horizontal distance of the building blocks relative to the body at the instant of peak net total moment. Despite these changes in lifting behaviour, the investigated ergonomic interventions still reduced the net total low back moment (by 4.9 (SD 2.0) Nm/kg when block mass was reduced and 53.6 (SD 41.0) Nm when initial lifting height was increased).

EFFECT OF SHIP MOTION ON LOW BACK MOMENTS DURING MANUAL LIFTING

INTRODUCTION Performing heavy work (such as lifting) on a ship is associated with a high prevalence of musculoskeletal problems [1,2]. Lifting results in high low back loading [3], thereby affecting low back pain risk. Ship motion may have an additional effect on low back loading during lifting [4]. Depending on the timing of a lifting movement, ship motion can either increase (for example when a ship accelerates upwards) or decrease (for example when a ship accelerates downwards) low back loading. The aim of the present study was to find out to what extent ship motion affects the peak low back moment at the L5S1 joint during manual lifting on a ship at sea. Furthermore, it was investigated whether people are able to time their lifting movements in such a way that the effect of ship motion reduces peak low back moments.

The effect of ship accelerations on three-dimensional low back loading during lifting and pulling activities

Manual materials handling on a moving platform, like a ship, might be a risk factor for the development of low back pain due to the influence of accelerations on low back loading. In the current simulation study, 3-D accelerations, measured on a frigate were applied to the kinematics of symmetrical and asymmetrical lifting movements and to a pulling task that had been performed under stable conditions. The aim was to find out to what extent low back loading is increased when the task execution is not adapted to the ship accelerations. Unfavorable timing, analyzed using the 99th percentile of predicted low back moments, resulted in only a moderate (up to 15%) increase of extending and total low back moments, and in a substantial increase of the twisting moment (up to 67%) during asymmetrical lifting. Moments in the pulling task were low and were relatively unaffected by ship accelerations, but adaptation of the movement pattern to prevent falling would be needed more often than during lifting. It furthermore appeared that a substantial reduction of low back loading by favorable timing is not a realistic option. Designing tasks in such a way that they are located midship would reduce the 99th percentile of predicted low back moments. During lifting, orienting the task in such a way that the feet are pointing sideward relative to the ship reduces the predicted peak twisting moment at the low back compared to pointing the feet forward or backward. Relevance to industry Accelerations on a ship may influence safety and loading of workers during manual materials handling. This study investigates potential effects of moderate accelerations on low back loading during lifting and pulling tasks. It is shown that working midship reduces the risk of low back overloading, compared to working at the front deck.

Biomechanical differences between best and worst performances in repeated free asymmetrical lifts

The purpose of the study was to evaluate, in 10 novice workers, the effect of the free practice of asymmetrical lifts with 3 different 15-kg containers. Practice effects were evaluated during one session (trials 1, 25 and 49), and also one month later (trial 50); containers effects were evaluated for homogeneity (2 boxes: homogeneous vs. heterogeneous) and for format (homogeneous box and cylinder). Each subject performed 150 lifts (50 practice trials and 3 containers per trial) but only 12 lifts were first analyzed. The data were obtained from 4 video cameras and a large force plate. A 3D dynamic rigid body model was used to evaluate low-back kinetics and kinematics. There were not any significant differences between the practice trials nor the boxes but there was high variability of performance within and between subjects. The main objective was then to evaluate the variability of these trials by contrasting the worst and best trials using three safety criteria: mechanical work, back efforts and asymmetry. The best strategies of reduced mechanical work (mean difference: 31%) was mainly associated with reduced knee flexion at take-off; strategies of reduced low-back moments at take-off (27%) were associated with smaller knee flexion and asymmetrical trunk efforts but larger feetspacings; finally, strategies of reduced back asymmetry at take-off (155%) were associated with reduced asymmetry of posture i.e. a better parallelism between shoulders/pelvis/handgrips and shoulders more parallel to the ground. Conclusions based on analyses of single trials or even means may be misleading. Relevance to industry Analyses of handling strategies including best and worst performances on the basis of specific safety criteria may help to better understand risk factors and suggest potential solutions for safer handling.

Low-back stresses when learning to use a materials handling device

This study examines the potential effect of short-term practice on low-back stresses during manual lifting and lowering of a 15 kg load, and while using two different types of materials handling devices (MHDs) to lift and lower a 40 kg load. The two MHDs used were an articulated balance arm and a pneumatic hoist. The expectation was that low-back dynamic moments, EMG measured torso muscle antagonism, and EMG predicted L4/L5 disc compression forces would rapidly decrease with practice, and that the manual lift-lower activities would be learned faster than the MHD-assisted exertions. Four naïve male college age subjects performed 40 lift and lower exertions, both manually and with the two MHDs for a total of 24 experiments. Non-linear regressions of the peak and average low-back moments, EMGs and disc compression values revealed only small decreases in the values (from 2 to 14%) over the 40 trials, and it was only statistically significant for five of the 48 regressions. This would seem to indicate that if learning is present in these tasks it is going to be very slow learning, and thus future studies will need to include a much larger number of trials. The effects of MHDs on the learning rates when compared to manual lifting learning rates was not statistically significant. It was shown, however, that MHDs had a particularly beneficial effect on reducing L4/L5 compression forces during load lowering activities despite the MHD load being much heavier than the manual load. It also was found that the level of torso muscle co-contraction increased significantly (2–4 times) when MHD handling was involved compared to manual lifting and lowering.

Frequency response of spine extensors during rapid isometric contractions: effects of muscle length and tension

During muscle contraction, electrical activity necessarily precedes force output, yet models that utilize processed electromyograms sometimes predict force as preceding EMG under rapid ballistic loading conditions. The purpose of this study was to define the frequency response transfer function of the upper and lower erector spinae musculature, at different lengths and tensions, using rectified, low pass filtered EMG. This would enable accurate estimates of force from the processed electromyogram, specifically during impulsive contractions. Abdominal and erector spinae EMG were measured in synchrony with impulsive low back moments in five men. EMG signals were rectified and low pass filtered repeatedly with cut-off frequencies from 1 to 3 Hz at 0.5 Hz increments in order to quantify the frequency response. It was found that EMG signals processed through a simple, Butterworth low pass filter could not produce the measured force output without an additional time shift. These shifts were quantified by cross-correlating EMG and force with increments of 1 ms. In order to define the transfer function of EMG to force, optimal cut-off frequencies were selected two ways: quantitatively by searching for maximum cross correlations coefficients, and qualitatively. Results indicated that the frequency response of both the upper and lower erector spinae can be modelled with a cut-off frequency between 2 and 2.5 Hz and that these values are not significantly modulated by changes in muscle length or tension.

Muscle activity and low back loads under external shear and compressive loading.

This study analyzed anatomic and neural control characteristics of the trunk musculature. Subjects were exposed to external shear and compressive loads with equivalent moments to evaluate activation patterns and loading on the low back. The migration of activity between the thoracic and lumbar erector spinae muscle groups was examined to determine whether the motor control system chooses to minimize joint loading by recognizing differences in moment, compression, and shear support requirements and assigning muscle activation in the most appropriate way. Loads were applied either parallel or perpendicular to the low back to create compressive or shear forces. No previous study has attempted to isolate the response of the trunk musculature with the type of external load. Eleven male subjects isometrically held an external load that was altered to create either a compressive or an anterior shear load on the low back but with equal extensor (reaction) moments (experiment 1). In a second experiment four men repeated the task with an increased range of applied loads (5-25 kg) together with measurements of intra-abdominal pressure. The tasks with a compressive external load resulted in significantly higher levels of activation for all seven electromyographic channels recorded. Intraabdominal pressure, compressive and shear joint forces were all higher in the compression loading method when equal loads and low back moments were compared. It was concluded that the motor control system does not arrange muscle activation levels in a way to minimize lumbar spinal loading at least for the relatively low levels of this study. Biomechanical models that use the objective criterion of minimum joint load may not be representative of the motor control system, at least in the low back.

Phase lag of the erectore spinae muscle response during whole body vibration

Although sneezing is known to induce low back pain, there is no objective data of the load generated when sneezing. Moreover, the approaches often recommended for reducing low back pain, such as leaning with both hands against a wall, are not supported by objective evidence.Participants were 12 healthy young men (mean age 23.25 ± 1.54 years) with no history of spinal column pain or low back pain. Measurements were taken using a three-dimensional motion capture system and surface electromyograms in three experimental conditions: normal for sneezing, characterized by forward trunk inclination; stand, in which the body was deliberately maintained in an upright posture when sneezing; and table, in which the participants leaned with both hands on a table when sneezing. We analyzed and compared the intervertebral disk compressive force, low back moment, ground reaction force, trunk inclination angle, and co-contraction of the rectus abdominis and erector spinae muscles in the three conditions.The intervertebral disk compressive force and ground reaction force were significantly lower in the stand and table conditions than in the normal condition. The co-contraction index value was significantly higher in the stand condition than in the normal and table conditions.When sneezing, body posture in the stand or table condition can reduce load on the low back compared with body posture in the normal sneezing condition. Thus, placing both hands on a table or otherwise maintaining an upright body posture appears to be beneficial for reducing low back load when sneezing.

Dynamically and statically determined low back moments during lifting : McGill SM, Norman RW. J Biomech 1985; 18: 877–885

Dynamically and statically determined low back moments during lifting : McGill SM, Norman RW. J Biomech 1985; 18: 877–885

Dynamically and statically determined low back moments during lifting

Assessment of the effects of lifting on the low back has most frequently been done with the aid of static models. Many lifting movements appear to have substantial inertial components. It was of interest, therefore, to determine the size of the difference between statically and dynamically calculated lumbar moments during a demanding but not unusual manual lift observed in a metal fabrication industry. The results of several trials by four young men showed that the dynamic model resulted in peak L4 L5 moments 19% higher on average, with a maximum difference of 52%, than those determined from the static model. The technique adopted in the lift could minimize the difference. When the inertial forces of the load itself and the load weight were incorporated into an otherwise static model (quasi-dynamic) then the resulting L4 L5 moments exceeded those of the fully dynamic model by 25%. In many industrial tasks static analyses may severely underestimate the demands of dynamic lifts. These results show that a reasonably inexpensive approach in lifting task analysis is to measure the dynamic forces of the load on the hands and to use these in an otherwise static model. This results in a conservative assessment of the injury risk of lifts at least of the type reported in this study.